1. Field of the Invention
The present invention relates to an apparatus for annealing a semiconductor material by means of irradiation with a laser beam.
2. Description of Related Art
Techniques for crystallizing amorphous silicon films by irradiating them with a laser beam have been known. Other techniques have been known wherein irradiation with a laser beam is performed to recover crystallinity of a silicon film which has been damaged as a result of implantation of impurity ions and to activate implanted impurity ions.
As a typical example of the latter kind of techniques, a technique has been known wherein regions which are to become a source and a drain of a thin film transistor are annealed by irradiating them with a laser beam after implanting impurity ions which are typically phosphorus or boron.
Such a process utilizing irradiation with a laser beam (generally referred to as xe2x80x9claser processxe2x80x9d) is characterized in that it causes substantially no thermal damage to a substrate. This is because a method utilizing irradiation with a laser beam only instantaneously heat the irradiated surface and the effect of the heating is not extended to the substrate.
This feature of causing no thermal damage to a substrate is important in fabricating active matrix type liquid crystal displays which recently have an expanding range of application.
There are demands for use of glass substrates as substrates of active matrix type liquid crystal displays from the viewpoint of cost and needs for such displays with a larger surface area.
However, a glass substrate can not withstand a heating process at temperatures as high as 600xc2x0 C. or more or 700xc2x0 C. or more. One effective technique for avoiding this problem is to perform the crystallization of a silicon film and the annealing after implantation of impurity ions as described above utilizing irradiation with a laser beam.
According to a method utilizing irradiation with laser beams, even if a glass substrate is used, there is substantially no thermal damage to the glass substrate. It is therefore possible to fabricate a thin film transistor having a crystalline silicon film even with a glass substrate.
However, since the area of a laser beam is small, a laser process has problems including low efficiency in processing a large area and low homogeneity in processing a large area.
It is an object of the present invention to provide a technique for a laser process used in fabrication of semiconductor devices wherein homogeneous annealing can be performed on a large area.
FIGS. 1(A) and 1(B) show an example of a laser radiation apparatus that employs the present invention. In FIGS. 1(A) and 1(B), 101 designates a laser oscillator which oscillates a laser beam by decomposing a predetermined gas using high frequency discharge to realize a state referred to as xe2x80x9cexcimer statexe2x80x9d.
For example, a KrF excimer laser oscillates a laser beam by means of high frequency discharge using Kr and F as material gases.
102 through 105 designate homogenizers. A homogenizer is constituted by a set of cylindrical lenses. The homogenizers 102 and 105 have a function of splitting a laser beam oscillated by the laser oscillator into parallel beams in a vertical direction to perform optical correction in the vertical direction.
The optical correction in the vertical direction contributes to homogenization of the energy density of a laser beam in the direction of the width of a line into which the laser beam is ultimately shaped.
Further, the homogenizers 103 and 104 have a function of splitting a beam in a horizontal direction to perform optical correction in the horizontal direction.
The optical correction in the horizontal direction contributes to homogenization of the energy density of a laser beam in the longitudinal direction of a line into which the laser beam is ultimately shaped.
106 designates a lens for controlling focusing of a laser beam in the horizontal direction. The lens 106 contributes to focusing of a linear laser beam in the longitudinal direction thereof.
107, 108 and 110 designate a lens system for controlling focusing of a linear laser beam in the direction of the width thereof. The primary function of this lens system is to shape the ultimately radiated laser beam into a linear configuration. 109 designates a mirror. A laser beam reflected by the mirror 109 is ultimately directed to a surface to be irradiated 111 through the lens 110.
For example, the surface to be irradiated 111 is a surface of an amorphous silicon film or a surface of a crystalline silicon film on which crystallinity is to be enhanced.
What is important is the setting of optical parameters of the homogenizers 103 and 104 for controlling the distribution of the radiation energy density of a laser beam in the horizontal direction (which corresponds to the longitudinal direction of the linear laser beam).
In general, variation occurs in the radiation energy density in the longitudinal direction of a linear laser beam unless the optical parameters of the homogenizers 103 and 104 are properly set.
The present invention is characterized in that variation in the radiation energy density in the longitudinal direction of a linear laser beam is corrected by optimizing the optical parameters of the homogenizers 103 and 104.
A set of the homogenizers 102 and 105 is provided in a different direction than another set of the homogenizers 103 and 104.
FIGS. 3(A) and 3(B) show photographs of a surface of a crystalline silicon film obtained by irradiating an amorphous silicon film with a laser beam.
FIG. 3(A) shows the result of annealing performed by forming the homogenizer 104 in FIGS. 1(A) and 1(B) using twelve cylindrical lenses having a width of 5 mm.
FIG. 3(B) shows the result of annealing performed by forming the homogenizer 104 in FIGS. 1(A) and 1(B) using five cylindrical lenses having a width of 6.5 mm.
FIG. 2 is an enlarged view of the homogenizer indicated by 104 in FIGS. 1(A) and 1(B). The homogenizer 201 is constituted by a plurality of cylindrical lenses 202.
Importantly, the number of the cylindrical lenses in the direction of the width of a laser beam incident upon the homogenizer is 7 or more, preferably 10 or more. The direction of the width of the laser beam must coincide or substantially coincide with the longitudinal direction of the line into which the laser beam is ultimately shaped.
Further, the width xe2x80x9caxe2x80x9d of the cylindrical lenses 202 in FIG. 2 must be 5 mm or less. Again, the direction of this width must coincide or substantially coincide with the longitudinal direction of the line into which the laser beam is ultimately shaped.
The length of the linear laser beam used for the annealing that provided the result as shown in FIG. 3(A) was 12 cm in its longitudinal direction. Any change in the length of the laser beam in the longitudinal direction still results in a difference in the effect of annealing as shown in FIGS. 3(A) and 3(B).
Homogeneous annealing as shown in FIG. 3(A) can be achieved when the above-described conditions are satisfied.
If there is any deviation from the above-described conditions, a vertically extending stripe pattern will be observed as shown in FIG. 3(B). This stripe pattern originates from variation in the radiation energy density of the linear laser beam in the longitudinal direction thereof.
The horizontally extending stripe pattern in the photograph (horizontal stripes) is variation caused during irradiation with a linear laser beam which is being scanned and is simply attributable to insufficient compliance to the conditions for radiation.
The difference in the effect of annealing as indicated by FIGS. 3(A) and 3(B) is attributable to improper setting of the optical parameters of the homogenizer 104.
According to one aspect of the present invention, there is provided an apparatus for radiating a linear laser beam characterized in that the width of cylindrical lenses forming a homogenizer for controlling the distribution of energy density of the linear laser beam in the longitudinal direction thereof is in the range from 0.1 mm to 5 mm.
According to another aspect of the invention, there is provided an apparatus for radiating a linear laser beam characterized in that the length (mm) of the linear laser beam in the longitudinal direction thereof and the number of cylindrical lenses forming a homogenizer for controlling the distribution of energy density of the linear laser beam in the longitudinal direction thereof are in the range defined by coordinates represented by (100, 7), (700, 50), (700, 140), and (100, 20).
The above-described coordinates are shown in FIG. 7. The relationship shown in FIG. 7 indicates that high homogeneity is achieved when the length of the linear laser ultimately radiated in the longitudinal direction thereof that corresponds to one cylindrical lens is generally in the range from 5 mm to 15 mm.
According to another aspect of the present invention, there is provided an apparatus for radiating a linear laser beam characterized in that it comprises a homogenizer for controlling the distribution of energy density of the linear laser beam in the longitudinal direction thereof and in that the width (mm) of the laser beam incident upon the homogenizer corresponding to the longitudinal direction and the width (mm) of cylindrical lenses forming the homogenizer are in the range defined by coordinates represented by (30, 0.1), (80, 0.1), (80, 5), (50, 5) and (30, 3).
The above-described coordinates are shown in FIG. 8. The relationship shown in FIG. 8 satisfies a condition that the width of the laser beam incident upon the homogenizer is in the range from 30 mm to 80 mm; the laser beam is divided by the homogenizer into 10 or more beams; and the width of the cylindrical lenses is in the range from 0.1 mm to 5 mm.
The homogenizer has a configuration as indicated by 201 in FIG. 2 and is constituted by a multiplicity of cylindrical lenses indicated by 202.
For example, the homogenizer for controlling the distribution of the energy density of a linear laser beam in the longitudinal direction thereof is indicated by 103 and 104 in FIGS. 1(A) and 1(B).